Various optical discs are known in the art including recordable optical discs of the magneto-optical recording and other modes and read-only optical discs such as compact discs. Information is recorded in and reproduced from the optical discs by directing laser light to the recording layer to detect reflected or transmitted light. Since recording and reproducing light often passes through disc substrates, the substrates are required to be transparent to the light.
Additionally, optical discs are characterized by an extremely high density of recording. Then any distortion or dimensional change of substrates can alter recorded signals. Thus the substrates are also required to be durable and low moisture absorptive. Polycarbonate resins satisfy these requirements and are amenable to mass production and thus cost effective. In fact, they have been used for the manufacture of compact discs and magneto-optical recording discs. Polycarbonate resin substrates, however, can sometimes lose optical uniformity due to internal residual stresses.
Since optical discs are generally driven for rotation by magnet clamping means, magnetic hubs must be attached to plastic substrates around their center hole. Particularly when hubs are fusion welded by a ultrasonic Or RF welding technique, internal strains are induced in an inner peripheral portion of the substrates, resulting in increased circumferential variations in DC level of reproduced signals, that is, an increase of DC modulation.